Add separate gravitational and centrifugal acceleration

[leouieda]

The Ellipsoid.normal_gravity methods calculates the combined gravitational and centrifugal acceleration. It would be nice to have these two components in separate methods (then normal_gravity can add them). When synthesizing data from spherical harmonics, there is no point in including the centrifugal component only to remove it afterwards.

[leouieda]

This is not possible for the ellipsoid as defined here. The Li & Goetze formula require constant gravity potential at the surface, hence separating the two components is not possible without rederiving all of the equations. That's beyond the scope of what we can do here.